Oil in Frame (OIF) refers to post 1971 Triumphs and BSAs when the separate oil tank for the dry sump Triumph/BSA 650 engines was replaced with a frame using a large diameter tube backbone type where the oil is carried inside the main large frame tube.

1973 and later OIF have a lower seat height. The big backbone of the frame makes for a fine handling bike. They are less popular than the earlier models (i.e. a bit cheaper) but by no means a bad bike. Plenty of them about too.

I have a Norton spin on filter and a Hyde 'bog brush' cooler, which adds plenty to capacity. The 'running hot' problem occurs after Ye Olde Oil leakes have allowed a good portion of the lubricant to escape to atmosphere, sure one can top it up again at each stop, but that requires carrying a bottle of Castrols finest with you, either that or 'sort out the leaks'....